High speed process for roasting coffee

ABSTRACT

A coffee roasting process for forming a low density roasted coffee having a yield of high soluble solids in which the green coffee beans are dry roasted by passage in a fluidized bed through a two-stage roaster. In the first stage, the beans are heated to a temperature of 550° F. to 570° F. by a roasting gas for partial roasting and expansion of their cellular structure. Then, in the second zone, the beans are contacted with an independent roasting gas stream at the same temperature and lower velocity to complete the roasting process.

The invention relates to a two-stage continuous roasting process forcoffee in which the roasted coffee product is of low density and highyield.

Green coffee is roasted to produce the desired dark brown color and toalter the natural elements present in the green beans to provide thearomatic qualities and taste or flavor values desired in a freshlybrewed coffee beverage. One known technique for the continuous roastingof coffee is disclosed in Nutting et al U.S. Pat. No. 3,572,235. There,the coffee beans are first subjected to steam in a pre-heating zone andthereafter progressively moved as a fluidized bed through a roastingzone in which the pre-heated beans are contacted with a hot roasting gasat a temperature in the range of 450° F.-575° F. Then, the beans aredried by contact with an ambient air stream.

It is well known that the price of coffee has increased significantly inrecent years. For economy, it is desirable to increase the yield pervolume of green coffee beans assuming that the quality of coffee couldbe maintained.

In Hubbard et al U.S. Pat. No. 4,169,164, a two-stage fluidized bedroasting process is disclosed to form an excellent roasted product oflow density and high soluble solids content. In the disclosed preferredprocess, the coffee beans are roasted in the first stage at atemperature of 450° F. to 470° F., while in the second stage they areroasted at about 540° F., with a fluidized bed depth of about 2 inches.

It is an object of the invention to provide a high-speed continuousroasting process in which the desired flavor and aroma characteristicsare developed from the green coffee beans to produce low density roastedcoffee product with a high yield of soluble solids.

Additional objects and features of the invention will appear from thefollowing detailed description taken in conjunction with theaccompanying drawings.

FIG. 1 is a schematic representation of apparatus suitable forperforming the continuous roasting process of the present invention.

FIG. 2 is an enlarged view in transverse section of the roaster of FIG.1, taken along the line 2--2.

FIG. 3 is an enlarged view of the roasting apparatus of FIG. 2 takenalong the line 3--3 of FIG. 2.

In general, the present process is directed toward a high-speed,two-stage roasting process carried out under conditions to form lowdensity roasted coffee product of high soluble solids yield. The beansare progressively and continuously moved and agitated through the tworoasting stages in a fluidized bed and thereafter cooled and ground toform the final product.

Referring specifically to the first stage or zone of the two-stageroasting process, green coffee beans are subjected to a high velocityheated roasting gas to effect at least a partial roasting of the coffeebeans. It has been found that by controlling the conditions in thisfirst zone, the cellular structure of the coffee beans is expanded. Theroasting gases are passed into the coffee beans as high velocity streamsof roasting gas which pass through the bed of beans at a sufficient rateto induce fluidization of the beans into a bed to provide uniformheating of the beans.

As set forth above, the heating conditions in the first zone aremaintained to provide an expansion of the cellular structure of thecoffee beans. This, in turn, produces a disrupted or slightly puffedcondition to the beans which forms porous openings or passageways fromthe surface of the beans to the interior of the same. This expandedstructure permits a highly efficient penetration or entry of roastinggases into the coffee beans during roasting and also provides more readyaccess to the soluble solids content of the beans by the action ofheated water during brewing of the coffee beverage. In this manner, theyield of the product is increased.

A major advancement of the present process over that of U.S. Pat. No.4,169,164 is a significant reduction in the total residence time in thefirst and second roasting chambers to about 3 to 4 minutes. This resultsin a significant increase in coffee output from the same roaster.Unexpectedly, this result is accomplished by simultaneous modificationof the temperature and bed depth of fluidized coffee bean during theroasting process. A bed depth suitable to accomplish the desired amountof heating in the first and second stages in a high-speed process isfrom about 0.25 in. to about 1.0 in., and preferably on the order of 0.5in.

The temperature of the roasting gas is adjusted to accommodate theshallower depth and thus higher speed. Suitable gas temperatures forboth zones are on the order of 550° F. to 570° F., preferably about 560°F., with a temperature variance between the first and second zones nogreater than about 5° F.

The flow rate of the roasting gas at this temperature for a givenresidence time and bed depth determines the heat input to the beans. Asuitable air flow in this stage is on the order of from about 19,000 to21,000 standard cubic feet per minute.

After passage through the first stage or zone, the fluidized bed ofcoffee beans move into the second zone in a bed of constant depth. Inthe second zone, the roasting conditions are substantially altered.There, roasting is completed under conditions to stabilize the openporous structure of the beans and to darken them into the desired darkbrown color without burning. In addition, the conditions are adjusted toform low density roasted coffee product of high soluble solids yield. Itis desirable to reduce the roasting gas velocity so that the total heatinput to the coffee beans in the second stage is insufficient to causeburning. A suitable roasting gas velocity is less than about 18,000cubic feet per minute and preferably on the order of 17,000 cubic feetper minute or less.

After completion of roasting in the second zone, a continuous stream ofthe roasted coffee beans is cooled rapidly by a suitable coolingtechnique to arrest the roasting operation and maintain the opencellular porous structure of the coffee beans. A suitable technique forsuch rapid cooling is by directing (e.g., by flowing or drawing), anambient air stream through a fluidized bed of the roasted coffeeproduct. Cooling is typically completed in a relatively short period oftime, ranging from 0.5 to 10 minutes and generally from 0.5 to 2minutes. Such cooling is preferably performed by employing streams ofhigh velocity ambient air which pass through the roasted beans at a ratesufficient to fluidize the same in a manner similar to that of thefluidized roasting process on the first and second zones. Cooling inthis manner is performed in less than about 2 minutes.

FIG. 1 schematically illustrates apparatus for carrying out thecontinuous roasting process of the present invention. Green coffee beansare introduced to the system at 10 to a suitable feed hopper 12 and aredirected onto an inclined chute or ramp 14 with the result that thebeans are force fed by gravity into the two-stage roaster unit 16.

Roaster 16 operates to provide a controlled oxidizing heat treatment ofthe green coffee beans. High velocity streams of heated roasting gasfunction to fluidize the beans within the roasting unit 16 and therebyto provide a uniform, even heat treatment. The roaster apparatusincludes a gas impervious bed forming plate 20 mounted for slidingvibratory movements with respect to the main housing 22 by means ofparallelogram links 24 pivotally connected to the support frame 26. Theplate 20 is vibrated in a conventional manner by a rotary eccentric 28driven by motor 30 connected to the plate by means of a connector link32. The apparatus is of the type illustrated in the aforementioned U.S.Pat. No. 3,572,235. It includes a pattern of nozzles or tubes 34 whichare suspended from a partition 36 which separates an upper plenum withinthe roaster housing from an exhaust chamber 40. Referring specificallyto FIG. 3, the plenum is divided into a first roasting zone 40 and asecond roasting zone 42 by an impermeable partition 44 which extendsfrom the top to the bottom of the roasting chamber in a verticaldirection to provide a barrier against air flowing between zones 40 and42. The two zones are so designated in FIG. 3.

Referring to the second roasting zone 42, fans or blowers 46 areprovided on opposite sides of the zone to control the flow of air intothe upper plenum chamber and through tubes 34 to achieve a high velocityjet action below each tube, illustrated at 48, which serves to suspendthe coffee beans in a fluidized state as they progress through theroaster. As described in the foregoing patent, the fluidizing effect isachieved by jetting of the roasting gases downwardly onto the gasimpermeable plate 20 simultaneously with a rapid withdrawal of the gasesupwardly through the blower units, for example, as illustrated by thearrows 50 and 52. Major portions of the recirculated gases is returnedto the tubes 34 as indicated by the arrows 54, while a portion of thegases is simultaneously removed from the roaster through exhaust ducts56 as indicated by the arrows 58. As illustrated in FIG. 1, the blowerunits are suitably driven by motors or like drive units 60 which can becontrolled to provide the desired gas velocities within tubes 34.Referring to FIG. 3, independent fans or blower units 62 are provided atopposite sides of the first stage plenum chamber 40 driven by motor 64of a similar type to fans or blowers 46.

Referring again to the first zone, the circulating gases are heated toeffect the desired heat treatment for the roast. Heating of the roastinggases may be accomplished by any suitable means. As illustrated in FIGS.2 and 3, the roasting gases are directly heated by injection and burningof a fuel gas within the plenum space of roaster unit 16. Apparatus forthis purpose is illustrated in the form of injection nozzles 66 onopposite sides of partition 44. Burning of the fuel gas (i.e., naturalgas, propane or the like) within the plenum is accomplished byintroduction of combustible mixtures of the gas with combustion airthrough each of nozzles 66. The combustion gas is being distributed inseparate independent streams in the first and second plenum roastingzones 40 and 42, respectively, separated by partition 44. In thismanner, different roasting gas velocities are maintained in each of thetwo roasting zones.

After the roasting operation in roaster 16, the hot roasted beans aredischarged from the end of bed forming plate 20 into cooling unit 70 asillustrated in FIG. 1. Beans are received on a chute or ramp 72 which issimilar in construction to feed ramp 14 for the roaster unit 16. Theentire cooling unit may be similar in construction to the roaster unit,with the ramp 72 forming a part of a vibrated gas-impervious plate 74within the cooler. The plate 74 is supported on frame work 76 connectedby similar parallelogram linkage including links 78. After cooling inunit 70, the beans are collected in a suitable basin 80 for subsequentunit operations including grinding and packaging. Cooling gases (i.e.,ambient air) may be circulated within the cooling unit by a fan orblower which functions to direct cooling air downwardly through tubes inthe same manner as in the roaster.

In a typical operation, the beans are advanced onto ramp 14 at a rate sothat the progress of the beans through both stages of the roaster isaccomplished in a period of about 3 to 4 minutes, with equal residencetimes in each stage. The beans are progressively moved at a constantstate of fluidization induced by the circulating roasting gases toprovide a fluidized bed at a depth on the order of 0.5 inches. Asuitable feed rate may be on the order of 10,000 pounds of coffee perhour. In the first roasting zone, a typical temperature is on the orderof 560° F., at a roasting gas flow velocity induced by the fan of about20,000 cubic feet per minute. Here, the initial heating of the beans tothe roasting temperature is accomplished while the cellular structure ofthe beans is expanded to provide a network of interconnecting pores.

In the second zone of roasting below plenum 42, the temperature of theroasting gas remains substantially constant at about 560° F., with areduced flow velocity of on the order of 17,000 standard cubic feet perminute. The roasted beans continue to develop the desired darker browncolor without burning and without losing the expanded porous cellularstructure formed in the first zone. Thereafter, the coffee is passedthrough the cooling unit 70 in a fluidized bed of the same depth. There,the beans are subjected to a gas stream at a flow rate similar to thatin the roaster unit to provide rapid cooling. A typical residence timefor adequate cooling is on the order of 0.5 to 2 minutes. Such rapidcooling is believed to assist in maintaining the open cellular porousstructure of the coffee beans.

The roasted coffee product produced by the foregoing process is of lowbulk density (about 0.36 to 0.37 gm/cc), comparable to the productproduced by the process described in U.S. Pat. No. 4,169,164. Thetechnique employed for measurement is to tap a 250 cc graduated cylindercontaining 50 grams of sample for 250 strokes. The density of the tappedvolume is determined by dividing the 50 grams by the volume left in thegraduated cylinder. The product of the present process also has a highsoluble solids yield comparable to the 4,169,164 patent.

The present process produces a comparable product to that of U.S. Pat.No. 4,169,164, but at a significantly increased roaster rate (10,000lb./hr. compared to 7,200 lb./hr.) and lower residence (3 to 4 minutescompared to 5 to 10 minutes).

What is claimed is:
 1. A continuous high speed process for dry roastinggreen coffee beans comprising the steps of progressively andcontinuously moving and agitating dry green coffee beans in a fluidizedbed successively through first and second separate roasting chambers,said bed depth in both chambers being maintained below about one inch,the beans being uniformly contacted in the first chamber with a dryroasting gas stream which fluidizes said beans, said gas being at atemperature from about 550° F. to 570° F., and the gas effecting atleast a partial roasting of the beans and expansion of the cellularstructure of the beans, then passing the beans through a second chamberin which the beans are uniformly contacted with an independent dryroasting gas stream, which fluidizes said beans to complete the roastingprocess, said independent gas stream being at substantially the sametemperature as the gas stream in the first chamber, and then cooling thebeans to form a low density roasted coffee product having a yield ofhigh soluble solids.
 2. The process of claim 1 in which the temperatureof the gas streams in said first and second chambers vary by no greaterthan about 5° F.
 3. The process of claim 1 in which the temperature ofthe gas streams in said first and second chambers is on the order of560° F.
 4. The process of claim 1 in which the bed depth in said firstand second chambers is approximately 0.5 inch.
 5. The process of claim 1in which the total residence time in said first and second chambers isfrom about 3 to 4 minutes.
 6. The process of claim 5 in which there isapproximately equal residence time in the first and second chambers.